Examples of a lithographic printing plate precursor which is employed at present chiefly in the field of the small-scale commercial printing include (1) a direct-drawing type printing plate precursor comprising a hydrophilic image receiving layer provided on a water resisting support, (2) a printing plate precursor comprising a zinc oxide-containing image receiving layer (ink receptive layer =lipophilic layer) provided on a water resisting support, which is made into a printing plate by drawing images directly thereon by means of, e.g., a thermal printer, a dry laser printer or an ink jet printer and then processing the non-image area with a desensitizing solution, (3) an electrophotographic material comprising a zinc oxide-containing photoconductive layer provided on a water resisting support, which functions as a printing plate precursor to be made into a printing plate by forming images on the photoconductive layer in an electrophotographic process and then processing the non-image area with a desensitizing solution, and (4) a silver halide photographic material functioning as a printing plate precursor, which comprises a silver halide emulsion layer provided on a water resisting support.
In general the printing plate constituted of a water-wettable non-drawing area (water receptive area=hydrophilic area) and a water-unwettable drawing area (ink receptive area=lipophilic area) is used for lithography. However, the lithographic printing plate precursors (2) and (3) are each provided with a ZnO-containing hydrophobic layer. As far as the printing operation is carried out without giving any processing to the printing precursors after image formation, the printing ink adheres to the non-drawing area also to make normal printing impossible.
Therefore, it is necessary to desensitize the non-drawing area of the printing plate precursor prior to the printing operation, and thereby confer the water wettability thereon. The desensitizing solutions which have so far been proposed for such a purpose comprise cyanide-containing (process) solutions containing as the main component ferrocyanide or ferricyanide, and cyanide-free (process) solutions containing as their main component an ammine-cobalt complex, phytic acid (inositol hexaphosphate) or a derivative thereof, or a guanidine derivative.
However, those processing solutions are not wholly satisfactory. More specifically, the ferrocyanide or ferricyanide-containing process solutions, though they have advantages of high desensitizing power, water-receptive firm film formability and high film-forming speed, have also disadvantages that, when exposed to light, they are colored and deposit precipitates due to lability of ferrocyanide and ferricyanide ions to heat and light, thereby undergoing a drop in desensitizing power, and further cause various problems regarding environmental pollution, including disposal of waste solutions, because the cyanide ions (CN.sup.-) contained therein are detected as free cyanogen.
From consideration of those disadvantages, the latter cyanide-free processing solutions have been proposed, wherein the desensitizer such as an ammine-cobalt complex, phytic acid or a guanidine compound is contained as a main component. However, even these solutions are not wholly satisfactory as a desensitizing (process) solution applied to lithographic printing plate precursors. This is because they are slow in film-forming speed, as compared with the former cyanide-containing process solutions, and have a defect that they cannot form a water receptive film having high enough physical strength to withstand printing operations at the first etching system using a processor, thereby causing generation of background stain and plugging in dot gradation.
As is generally known, phytic acid and its metal derivatives form metal chelate compounds. Many compounds of such a kind have already been offered as desensitizers for offset printing plate precursors. However, they are each slow in film-forming speed and cannot form a water-receptive film applicable to printing at the first time of processing with a processor. Therefore, they have drawbacks of occurring poor ink separability, background stain and plugging in dot gradation.
In order to overcome such drawbacks, the addition of various additives to the processing solutions containing phytic acid compounds has been examined.
For instance, the combined use of phytic acid compounds and the metal complexes of aminocarboxylic acids (JP-B-2-39397, the term "JP-B" as used herein means an "examined Japanese patent publication"), the combined use of phytic acid compounds and the hexametaphosphates (JP-B-62-7597), and the addition of lower amines, alkanolamines or polyamines to the processing solutions of the foregoing type (e.g., JP-A-54-117201, JP-A-53-109701 and JP-A-1-25994; the term "JP-A" as used herein means an "unexamined published Japanese patent application") were considered. However, those processing solutions have two problems; one problem is in that, although they can provide good water receptivity in the early stage of use, their etching ability is lowered by continuous use to bring about a decline in water receptivity, and the other problem is in that their use after long-term storage causes deterioration in water receptivity to generate background stain.
The addition of cationic polymers to the processing solutions containing phytic acid compounds (JP-A-50-23099) produces, similarly to the addition of the foregoing additives, deterioration in properties of the processing solutions by the continuous use or long-term storage, and causes rust in some cases.
Further, the combined use of phytic acid compounds with the polyethyleneimine copolymers has been proposed (JP-A-7-68967 and JP-A-7-137475). However, it allows the processing solutions only a narrow latitude in the matter of compatibility of the etching ability for making the non-image area receptive of water with the ink receptivity of the image area, or cannot dissolve the problem of causing deterioration in properties by long-term continuous use.
In recent years, on the other hand, automatic printing machines, particularly miniaturized ones, of the type which incorporate a desensitizing system into the body thereof from the viewpoint of saving labor have been remarkably popularized, and the plate making of an offset master by electrophotographic system has enabled a reduction of processing time. Such a situation has been demanding a reduction in desensitizing time and an extension of the life span of a desensitizing solution.
Further, it has been proposed to adopt a digital exposure method in the electrophotographic system master making system also. Such an exposure method has enabled easy making of masters having high-definition images, such as middle tone and screen tint, in addition to conventional plate-making images constituted mainly of line original and letter original. As a result, it has been demanding to make a printing plate enabling the reproduction of such high-definition images on printing materials in a printing process. On the other hand, thermal and dry laser printers have made it possible to confer high definition on plate-making images and reduce background stains on non-image areas, and thereby a demand for making a printing plate from a printing plate precursor of direct-drawing type by prepress processing has been generated. However, it is difficult with conventional desensitizing solutions to meet the foregoing demands.